Bulbasaur is a small, quadrupedal Pokémon with blue-green skin marked by darker patches, red eyes, and a short, blunt snout. The most distinctive feature is the large green plant bulb on its back, which was planted there at birth. This bulb is central to its biology, as it stores energy and allows Bulbasaur to perform photosynthesis, enabling it to survive for days without eating. The bulb will glow blue when it is close to evolving, and from it, Bulbasaur can extend two long, slender, tentacle-like vines used for attacks and grabbing objects.

In the Pokémon anime, the most prominent Bulbasaur is the one owned by the main character, Ash Ketchum. This Bulbasaur was not obtained as a first partner from a Professor but was caught by Ash early in his journey in the Kanto region. Before joining Ash, Bulbasaur lived in a hidden village where a kind-hearted girl named Melanie cared for sick, injured, and abandoned Pokémon. Bulbasaur served as the village's bodyguard, protecting the other Pokémon. This experience shaped its personality significantly, making it cautious, stubborn, and initially hostile towards human Trainers, having witnessed the effects of abandonment firsthand. However, after battling alongside Ash to defend the village from Team Rocket, Bulbasaur grew to trust him. Demonstrating its proud and stubborn nature, it agreed to be caught only after Ash defeated it in a fair battle.

Throughout the series, Bulbasaur's personality is defined by its loyalty, maturity, and reliability. It is frequently portrayed as a level-headed leader and a peacekeeper among the other Pokémon in Ash's group and later among the large population of Pokémon at Professor Oak's laboratory, where it helps resolve disputes. Despite its serious demeanor and a cynical streak that sometimes surfaces, Bulbasaur has a nurturing instinct, especially towards younger and weaker Pokémon. In battle, it is fearless and tenacious, often refusing to give up even when at a disadvantage.

One of the defining moments in Bulbasaur's development occurs when it is given the opportunity to evolve into Ivysaur. During a special evolutionary festival in a hidden garden, a Venusaur and many Bulbasaur gather to evolve. Although its own bulb begins to glow and the process starts, Bulbasaur ultimately chooses to reject evolution and remain in its current form. This decision is driven by its strong sense of pride and its desire to forge its own path, a trait it has shown since its days as a bodyguard. Ash supports this choice, and from then on, Bulbasaur remains in its unevolved state, content with its identity. Notably, after this event, it learns the powerful Solar Beam attack, proving that evolution is not the only measure of strength.

Bulbasaur shares a close friendship with Ash's Squirtle, with the pair often being shown as a team. Their dynamic frequently contrasts Bulbasaur's serious, responsible nature with Squirtle's more boisterous and goofy personality. Throughout Ash's journey through the Kanto and Orange Islands regions, Bulbasaur was a vital member of his team, winning many gym battles and league matches. Its notable abilities include the Vine Whip, Razor Leaf, and Leech Seed attacks, as well as the aforementioned Solar Beam. It also proves resourceful outside of battle, using its vines to rescue people and Pokémon, carry objects, and even tickle opponents.